The invention relates to a method for producing a metal-ceramic substrate including a first and second metallization and at least one ceramic layer accommodated between the first and second metallization, wherein first and second metal layers and the at least one ceramic layer are stacked superposed in such a way that free edge sections of the first and second metal layers respectively project beyond the edges of the at least one ceramic layer forming projecting free edge sections.
Metal-ceramic substrates in the form of printed circuit boards comprising a ceramic layer and at least one metallization connected to a surface side of the ceramic layer and structured for the formation of strip conductors, contacts, contact faces or terminal faces are known in the most diverse embodiments. Such metal-ceramic substrates are used for example for constructing power semiconductor modules, i.e. are intended for higher operational voltages, namely 600 V and more. One of the requirements on such power semiconductor modules is a sufficiently high partial discharge resistance, wherein metal-ceramic substrates also have to meet this requirement.
Furthermore, the so-called “DCB process” (“Direct-Copper-Bonding”) is known for connecting the metallization-forming metal foils or metal layers to one another or to a ceramic substrate or a ceramic layer. Metal layers, preferably copper layers or copper foils, are connected to one another and/or to a ceramic layer, namely using metal or copper sheets or metal or copper foils, which at their surface sides comprise a layer or coat (“fusing layer”) of a chemical compound of the metal and a reactive gas, preferably oxygen. In the case of this process described by way of example in US-PS 37 44 120 or in DE-PS 23 19 854, this layer or this coat (“fusing layer”) forms a eutectic with a melting temperature below the melting temperature of the metal (e.g. copper), so that by placing the metal or copper foil on the ceramic layer and by heating all the layers, the latter can be connected together, namely by melting of the metal layer or copper layer essentially only in the region of the fusing layer or oxide layer. One such DCB process comprises for example the following process steps:                oxidation of a copper foil in such a way that a uniform copper oxide layer results;        placing the copper foil with the uniform copper oxide layer onto the ceramic layer;        heating of the composite to a process temperature between 1025 to 1083° C., for example to approx. 1071° C.;        cooling to room temperature.        
A drawback of the DCB process consists in the fact that process-related imperfections can arise between the respective copper layer and the ceramic layer. Indeed, these imperfections scarcely impair the thermal properties of a metal-ceramic substrate produced using the DCB process, but an impairment of the partial discharge resistance of the power semiconductor module produced therefrom results.
Furthermore, the so-called active soldering method for connecting metallization—forming metal layers or metal foils, in particular also copper layers or copper foils, to a ceramic material or a ceramic layer is known from documents DE 22 13 115 and EP-A-153 618. With this method, which is also used especially for the production of metal-ceramic substrates, a connection is produced between a metal foil, for example copper foil, and a ceramic substrate, for example an aluminium nitride ceramic, at a temperature between approx. 800-1000° C. using a hard solder, which also contains an active metal in addition to a main component, such as copper, silver and/or gold. This active metal, which is for example at least one element of the group Hf, Ti, Zr, Nb, Ce, produces a connection between the hard solder and the ceramic by chemical reaction, whereas the connection between the hard solder and the metal is a metallic hard solder joint. A drawback, however, is that the required hard solder is very cost-intensive and the structuring of the metallization applied by means of an active soldering method is costly from the process standpoint.
A method of producing a stack comprising a plurality of metal plates for forming a cooler is already shown in EP 1 716 624 B1, wherein a plurality of thin metal plates or metal foils are connected to one another to form a stack, which then form a cooler, in particular a micro-cooler. To prevent the formation of micro-cavities in the transition regions between the metal plates, a post-treatment of the plate stack takes place in a protective gas atmosphere at a high treatment temperature below the jointing temperature and at a high gas pressure in the region between 200 and 2000 bar. This post-treatment is also referred to as hot-isostatic pressing (“HIP process”). The exposure of the plate stack to high gas pressure in the protective gas atmosphere under the stated temperature conditions leads, amongst other things, to the fact that the connection between the plates is for the most part free from micro-cavities, i.e. there are no recesses or holes in the connection region of two metal plates. As a protective gas, use is made here of nitrogen, argon or other inert or noble gases. The treatment temperature is adjusted such that diffusion bonding arises between the adjacent surfaces of the metal plates.
A method for producing a metal-ceramic substrate is known from EP 1 774 841 B1, wherein a copper layer is applied to at least one surface side of a ceramic layer using the DCB process. Here, the metal-ceramic substrate is subjected in a following process step to a gas pressure in the range from 400-2000 bar and is post-treated at a post-treatment temperature in the range between 450 and 1060° C. To prevent imperfections or micro-cavities in the region of the metal-ceramic transition following the DCB process, the substrate is subjected to the described post-treatment in a closed pressure chamber in a protective gas atmosphere, for example an argon atmosphere, by heating to a temperature of approx. 560° C. at a pressure of approx. 1100 bar. The bonding of the copper metallizations to the ceramic layer is thus enhanced and the formation of imperfections markedly reduced.
The aforementioned methods for eliminating imperfections have the drawback of a high process-related outlay, especially since, in the first place, a direct flat jointing connection between the metal layer and the ceramic layer has to be produced by means of a first connection technology and the latter subsequently has to be subjected to a post-treatment in order to eliminate micro-cavities arising in the jointing process.
A method for producing a direct flat connection of a ceramic layer to a metal layer is also already known from U.S. Pat. No. 4,763,828, wherein diffusion bonding in an argon protective gas atmosphere is described using the HIP process.
A multilayer casing material comprising a ceramic layer and a metallic material for nuclear reactors is known from WO 2012/048 071 A1. A tube or a channel is formed from this multilayer casing material. The metallic material forms the “inner” layer of the tube or the channel and brings about hermetic sealing of the latter.
Furthermore, a method for producing an electrical resistance comprising a metal foil connected to a ceramic substrate is known from U.S. Pat. No. 4,325,183, wherein a direct flat connection between the metal foil and the ceramic substrate is produced by means of the HIP process. For this purpose, the arrangement comprising the metal foil and the ceramic substrate is accommodated in a sealed envelope, wherein the envelope lies adjacent in a tight manner and has previously been evacuated.